1. Field of the Invention
The present invention relates to a glass-impregnated fiber-reinforced ceramic suitable for use in a hot oxidizing atmosphere, and a method of manufacturing the same.
2. Description of the Related Art
Ceramics have become watched as lightweight, highly heat-resistant structural materials in recent years owing to their excellent heat resistance superior to that of metals, and their high specific strength. However, there are still problems in using ceramics as structural materials in respect of reliability because ceramics have low toughness as compared with that of metals. Efforts have been made for the development of fiber-reinforced ceramic composite materials having high toughness as well as the excellent characteristics of ceramic materials.
Generally, the toughness of a fiber-reinforced ceramic (hereinafter referred to sometimes as xe2x80x9cFRCxe2x80x9d) is most greatly dependent on the bonding strength of the interface between the fibers and the matrix of the FRC. The toughness is high if the bonding strength of the interface between the fibers and the matrix is low, and is low if the bonding strength of the interface between the fibers and the matrix is high. Such a characteristic of toughness is considered to be due to a fact that energy is consumed in extracting the fibers from the matrix if the bonding strength of the interface between the fibers and the matrix is low. Accordingly, when producing an FRC having a high toughness, the bonding strength of the interface between the fibers and the matrix of the FRC is reduced properly by, for example, coating the fibers.
The fiber-reinforced ceramic has a ceramic matrix, and inorganic fibers, i.e., reinforcing material, embedded in the ceramic matrix, and the ceramic matrix and the inorganic fibers are chosen out of various materials. The following are representative FRC manufacturing methods.
(a) Method which molds a mixture of fibers and a ceramic precursor, namely, a substance which can be converted into a ceramic material by thermal decomposition, in a molding and fires the molding.
(b) Method which forms a structure only from fibers, impregnates the structure with a ceramic precursor, and fires the structure impregnated with the ceramic precursor (Japanese patent application unexamined laid open No. 8-157271).
(c) Method which forms a structure only from fibers, impregnates the structure with a molten ceramic material, and fires the structure impregnated with the molten ceramic material.
Fiber-reinforced ceramics having excellent heat resistance, high specific strength and high toughness as mentioned above are used widely for various purposes. However, the following problems arise when conventional FRCs are used a hot oxidizing atmosphere.
Referring to FIG. 5 showing a fiber-reinforced ceramic 1 manufactured by a conventional method in a typical sectional view, the fiber-reinforced ceramic 1 has a ceramic matrix 4, and inorganic fibers 3 embedded in the ceramic matrix 4, and the ceramic matrix 4 has voids 5 which develop unavoidably during a manufacturing process. If the inorganic fibers 3 are, for example, silicon carbide fibers, the ceramic matrix 4 is silicon carbide, and the FRC 1 is heated at 1200xc2x0 C. in the atmosphere, oxygen permeates the FRC through the voids 5, reaches the interfaces between the silicon carbide fibers and the silicon carbide matrix, and oxidizes the silicon carbide fibers and the silicon carbide matrix to produce SiO2 glass in the interfaces. The SiO2 glass formed in the interfaces bonds the silicon carbide fibers and the silicon carbide matrix firmly together, which reduces the toughness of the FRC 1 greatly. Therefore, the strength at elevated temperature of the FRC 1 in an oxidizing atmosphere or the strength of the same after the FRC 1 has been exposed to an oxidizing atmosphere is reduced greatly. Such a phenomenon is a problem common to FRC 1 consisting of easily oxidizable components. The notch effect of the voids 5 reduces the strength of the conventional FRC 1.
The conventional art method proposed to solve the foregoing problem in Japanese patent application unexamined laid open No. 8-157271 coats the surface of an FRC with glass. This method improves the oxidation resistance and the room temperature strength of the FRC by the sealing effect of the glass. However, the properties of the FRC manufactured by this method are still below the level of properties required of structural materials for use in a hot oxidizing atmosphere.
The inventors of the present invention found through elaborate studies of the foregoing problems that the foregoing problem can be solved by impregnating the conventional fiber-reinforced ceramic with glass so that the voids are filled up with the glass and have made the present invention on the basis of the knowledge acquired through the studies.
The present invention has been made to solve the foregoing problems and it is therefore an object of the present invention to provide a glass-impregnated, fiber-reinforced ceramic having excellent oxidation resistance in a hot oxidizing atmosphere and high room temperature strength.
Another object of the present invention is to provide a method of manufacturing a glass-impregnated, fiber-reinforced ceramic having excellent oxidation resistance in a hot oxidizing atmosphere and high room temperature strength.
According to a first aspect of the present invention, a glass-impregnated, fiber-reinforced ceramic including a ceramic matrix having voids opening outside; inorganic fibers embedded in the ceramic matrix; and a glass matrix with which the voids of the ceramic matrix are filled.
When the glass-impregnated, fiber-reinforced ceramic is placed in a hot oxidizing atmosphere, the ceramic matrix and the inorganic fibers embedded in the ceramic matrix are insulated from oxygen contained in the hot oxidizing atmosphere because at least the voids in the ceramic matrix are filled up with the glass matrix. Therefore, even if the ceramic matrix and the inorganic fibers are subject to oxidation, the oxidation of the ceramic matrix and the inorganic fibers are protected surely from oxidation and hence a main cause of the reduction of the high-temperature strength and the oxidation resistance of the glass-impregnated, fiber-reinforced ceramic is eliminated. Accordingly, the high-temperature strength and the oxidation resistance of the glass-impregnated, fiber-reinforced ceramic of the present invention are far higher than those of the conventional fiber-reinforced ceramic not impregnated with glass. Since the cause of reduction of the strength due to the notch effect of the voids can be eliminated by the densifying effect of the impregnation with glass, the room temperature strength of the glass-impregnated, fiber-reinforced ceramic of the present invention is far higher than that of the conventional fiber-reinforced ceramic.
The glass matrix may be one of AlPO4, PbO, ZnO, SiO2, B2O3, Al2O3, Na2O, CaO, MgO, Li2O and K2O or a combination of some of those substances. A material of an appropriate composition having an appropriate softening point can be used as the glass matrix according to the temperature of the atmosphere in which the glass-impregnated, fiber-reinforced ceramic of the present invention is to be used because the glass matrix may be one of those substances or a combination of some of those substances. Therefore, voids can be effectively stopped up with the glass matrix at a working temperature and the glass matrix is able to exercise its oxygen intercepting effect effectively in a wide temperature range.
According to a second aspect of the present invention, a method for manufacturing a glass-impregnated, fiber-reinforced ceramic, comprising the steps of: producing a fiber-reinforced ceramic including a ceramic matrix having voids opening outside and inorganic fibers embedded in the ceramic matrix; impregnating the fiber-reinforced ceramic with glass precursor solution containing glass precursor to fill the voids of the ceramic matrix with the glass precursor solution; and subjecting the fiber-reinforced ceramic impregnated with the glass precursor solution to heat treatment.
The fiber-reinforced ceramic can be surely and quickly impregnated with the glass precursor solution containing a glass precursor which can be vitrified by thermal decomposition by immersing the fiber-reinforced ceramic in the glass precursor solution under a vacuum atmosphere. When the fiber-reinforced ceramic thus impregnated with the glass precursor solution is subjected to a heat treatment, the glass precursor is vitrified by thermal decomposition to form the glass matrix in the voids. Thus, the glass-impregnated, fiber-reinforced ceramic can be efficiently manufactured by simple processes.
The impregnating step of the fiber-reinforced ceramic with the glass precursor solution and the heating treatment of the fiber-reinforced ceramic impregnated with the glass precursor solution may be alternately repeated several times. If the impregnating process and the heating process are repeated alternately several times, the number of voids formed by the dissipation of components of the glass precursor solution filling up the voids in the fiber-reinforced ceramic caused by the heating process decreases gradually to increase the void filling ratio of the glass matrix greatly. Consequently, the oxygen intercepting effect and the densifying effect of the glass matrix are further enhanced.
The producing step of the fiber-reinforced ceramic may include depositing ceramic material by a chemical vapor deposition (CVD) process on the inorganic fiber to form the ceramic matrix. The fiber-reinforced ceramic can be formed in a thin structure by thus depositing the ceramic matrix on the inorganic fibers.
According to a third aspect of the present invention, a method for manufacturing a glass-impregnated, fiber-reinforced ceramic, includes the steps of: producing a fiber-reinforced ceramic having a ceramic matrix having voids opening outside and inorganic fibers embedded in the ceramic matrix; and impregnating the fiber-reinforced ceramic with molten glass to fill the voids of the ceramic matrix with the molten glass.
When impregnating the fiber-reinforced ceramic with the molten glass, the fiber-reinforced ceramic is immersed in the molten glass in a vacuum atmosphere to stop up the voids in the fiber-reinforced ceramic. Therefore, the fiber-reinforced ceramic can be quickly and surely filled with the molten glass. Since the fiber-reinforced ceramic can be impregnated with the molten glass by a single process, the glass-impregnated, fiber-reinforced ceramic manufacturing method needs a small number of processes and the glass-impregnated, fiber-reinforced ceramic can be efficiently manufactured.
The impregnating step of the fiber-reinforced ceramic with the molten glass may be repeated for several times. If the impregnation process is repeated several times, the void filling ratio of the glass matrix increases greatly, which further enhances the oxygen intercepting effect and the densifying effect of the glass matrix.